1. Field of the Invention
The present invention relates to a radio communication apparatus and channel estimating method applied to a communication terminal apparatus and base station apparatus that perform pre-equalization used in a digital mobile communication system.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating a configuration of a conventional radio communication apparatus. Radio communication apparatus 10 illustrated in FIG. 1 is principally comprised of pre-equalization processing section 11, DA conversion section 12, radio section 13, antenna 14, AD conversion section 15 and channel estimating section 16.
A signal received at antenna 14 is subjected to the predetermined radio processing at radio section 13, and the resultant signal is converted into a digital signal at AD conversion section 15 to be output to channel estimating section 16. Channel estimating section 16 estimates a channel condition at the time of reception, i.e., a multipath condition from the digital signal, and outputs channel information indicative of the estimation contents to pre-equalization processing section 11. Pre-equalization processing section 11 performs pre-equalization described later on transmission data using the channel information. The pre-equalization-processed signal is converted into an analog signal at DA conversion section 12, and the analog signal is converted into a radio signal with a carrier frequency at radio section 13 to be transmitted from antenna 14.
With reference to FIGS. 2A to 2C, the pre-equalization will be described next that the conventional radio communication apparatus performs. A received multipath condition is assumed to be a condition in which preceding signal 21 and delayed signal 22 coexist as illustrated in FIG. 2A. In this case, channel estimating section 16 estimates preceding signal 21 and delayed signal 22, and outputs the channel information indicative of the estimation contents to pre-equalization processing section 11. Pre-equalization processing section 11 performs the pre-equalization so that the multipath condition of the received signal at a communication partner apparatus becomes a condition, as illustrated in FIG. 2B, in which preceding signal 21 is left as a desired signal with delayed signal 22 canceled. In addition, in FIG. 2B the broken line indicates the canceled delayed signal.
By thus performing the pre-equalization on the transmission data, the communication partner apparatus is not affected by the multipath at the time of reception, and thereby does not need to perform the equalization, whereby it is made possible for a receiving apparatus with a simple function to obtain received data.
In the radio communication, it is general that a received level of preceding signal 21 is larger than that of delayed signal 22 as illustrated in FIG. 2A. However, as illustrated in FIG. 2C, a case sometimes occurs that a received level of preceding signal 23 is extremely smaller than that of delayed signal 24 depending on propagation environments. In this case, the communication partner apparatus is capable of obtaining a better reception characteristic when the reception processing is performed on delayed signal 24 with a higher received level.
However, in the conventional radio communication apparatus, the channel estimation section always performs the channel estimation such that a preceding signal is a desired signal when a noise level is lower than a received level of the preceding signal, whereby the pre-equalization processing section performs the pre-equalization so as to cancel the delayed signal even in the case as illustrated in FIG. 2C. Therefore, there arises a case that a reception characteristic deteriorates in the communication partner apparatus.